EVGA GTX 770 SC frame rate issues?

I recently just installed Far Cry 3 about 3 days ago on my computer, and every time I have started the game, I was getting 60 FPS on ultra settings. However, when I launched the game tonight, it was hovering at around 30-45 FPS without me changing any settings for anything. I have actually had this same experience with games before. Like with Bioshock Infinite, some of the time the game would be running beautifully at 60 FPS when I started it, but at other times, it would run around 20-30 FPS. Would anyone happen to know what is going on? Also, my 770 is NOT overclocked.


System Specs:

CPU: Intel Core i5-4690K 3.5GHz Quad-Core Processor
Motherboard: Asus Z97-A ATX LGA1150 Motherboard
Memory: Kingston Fury Black Series 8GB (2 x 4GB) DDR3-1866 Memory
Storage: Seagate  1TB 3.5" 7200RPM Hybrid Internal Hard Drive
Video Card: EVGA GeForce GTX 770 2GB Dual Superclocked ACX Video Card
Case: Fractal Design Define R4 Blackout with Window ATX Mid Tower Case
Power Supply: EVGA 750W 80+ Gold Certified Fully-Modular ATX Power Supply
Monitor: Acer H236HLbid 60Hz 23.0" Monitor 
Wireless Network Adapter: TP-Link TL-WDN4800 802.11a/b/g/n PCI-Express x1 Wi-Fi Adapter
Keyboard: Cooler Master CM Storm Devastator Gaming Bundle Wired Gaming Keyboard w/Optical Mouse

did your gpu driver updated lately?

Nvidia had an update recently .

Probably that , that's why I wait to update .

Yes, I always make sure to download the latest Nvidia drivers when they come out.